Plasma-assisted cvd of carbonaceous films by using a bias voltage

ABSTRACT

Diamond films or I-Carbon films can be formed on a surface of an object by virtue of plasma-assisted chemical vapor deposition. The hardness of the films can be enhanced by applying a bias voltage to the object during deposition.

This is a continuation of Ser. No. 07/444,308 filed Dec. 1, 1989, nowabandoned, which is a divisional of Ser. No. 07/225,860 filed Jul. 9,1988, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a carbon material and a carbon depositionmethod, and more particularly, relates to a carbon deposit containing ahalogen and a fabricating method therefor.

Recently, ECR (Electric Cyclotron Resonance) CVD has attracted theinterest of researchers as a new method of manufacturing thin films,particularly amorphous thin films. For example, Matsuo et al disclosesone type of such as ECR CVD apparatus in U.S. Pat. No. 4,401,054. Thisrecent technique utilizes microwave energy to energize a reactive gassuch that it develops into a plasma. A magnetic field functions to pinchthe plasma gas within the excitation space. Within this excitationspace, the reactive fas can absorb the energy of microwaves. A substrateto be coated is located distant from the excitaion space (resonatingspace) for preventing the same from being spattered. The energized gasis showered onto the substrate from the resonating space. In order toestablish electron cyclotron resonance, the pressure in a resonatingspaces is kept at 1×10⁻⁵ Torr at which pressure electrons can beconsidered as independent particles and resonate with the microwaveenergy in an electron cyclotron resonance on a certain surface on whichthe magnetic field strength meets the requirement for ECR. The excitedplasma is extrated from the resonating space, by means of a divergentmagnetic field, and is conducted to a deposition space which is locateddistant from the resonating space and in which there is disposed asubstrate to be coated.

In such a prior art method, it is very difficult to perform carbondiposition of a polycrystalline or single-cryustalline structure, sothat currently available methods are substantially limited to processesfor manufacturing amourphous films. Also, high energy chemical vaporreaction can not be readily be accomplished by such a prior art andtherefore it has not been possible to form diamond films or other filmshaving high melting points, or uniform films on a surface havingdepressions and caves can not be formed. Furtheremore, it was impossibleto coat the surface of a super hard metal such as tungsten carbide witha carbon film. Because of this it is necessary to coat a super hardsurface with a fine powder of diamond for use of abrasive which has asufficient hardness and to make sturdy mechanical contact between thediamond powder and the substrate surface.

Furthermore, it is effective to deposit a hard film on a surface ofglass, plastic, metal, resin and so forth for the purpose of protectingthe surface from mechanical attachs such as abrasive or scratchingattachs. Films made of Al₂ O₃, TiN, BN, WC, SiC, Si₃ N₄ and SiO₂ andthose described in Japanese Patent Application No. Sho 56-146930.However, such conventional protecting films have high resistivities and,as a result, tend to generate static electricity which collects dust andfine particles on their surface from the surrounding atmosphere. On theother hand, when used in the application utilizing static electricity,the films aging is accelerated because of the electric chargeaccumulated on the films.

To avoid such a shortcoming, a conductive substance may be added intothe protecting films. In such a case, however, the added substance playsas the absorption center of the incident light so that the added filmscan not be used for application in which transmissivity of protectingfilms is required.

Still further, it is likely that such conventional films are peeled offbecause of accumulated internal stress depending on the depositioncondition. Accordingly, the thickness has to be reduced or anintermediate film having a high adhesivity has to be interposed betweenthe protecting film and the underlying surface.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an excellentcarbon material and a fabricating method therefor.

It is another object of the present invention to provide an excellentcarbon material having a high adhesivity.

It is a further object of the present invention to provide an excellentcarbon material whose characteristics can be easily controlled.

It is a still further object of the present invention to provide anexcellent carbon material accumulating little stress therein.

According to one aspect of the invention, in addition to a hydrocarbon,a halogen gas or halogen compound gas is introduced into the reactionchamber. Examples of halogen compound gases are fluorine compounds suchas NF₃, SF₃ and WF₆, chlorine compounds such as CCl₄, bromine compoundssuch as CH₃ Br and iodine compounds. The carbon material formed inaccordance with the present invention contains a halogen at 0.1-50 atom% which is controlled by adjusting the introduction rate of the halogencompound gas.

Among halogens, fluorine is most useful from the view point of avoidingcorrosion of the inner wall of the reaction chamber. The carbon compoundgas does preferably not include a halogen element, so that thepro-portion of halogen additive can be easily controlled. In case of thereaction between CH₄ and CF₄, radical carbon atoms are generated inaccordance with the below equation:

    CH.sub.4 +CF.sub.4 →2C+4HF

The conductivity, transparency and hardness of carbon material vary inaccordance with the proportion of halogen. In what follow, experimentalresults are described.

Carbon coatings were deposited using ethylene introduced at 10 SCCM andNF₃ introduced at varied rates. The pressure in a reaction chamber was10 Pa and input power was 0.08 W/cm². The relationship between theconductivity and the introduction rate of NF₃ in FIG. 1. Theconductivity increased as the rate increased. The relationship betweenthe transparency and the introduction rate is plotted in FIG. 2. Thetransparency increased as the rate increased. The relationship betweenthe hardness and the introduction rate is plotted in FIG. 3. Thehardness decreased as the rate increased. The decrease of the hardnessmeans the decrease of the inner stress.

As described above, the conductivity, hardness, transparency of filmsdeposited in accordance with the present invention can be easilycontrolled over wide ranges. An optimal property required for aparticular application can be attained at a relatively low productioncost. The proportion of a halogen can be controlled by changing theintroduction rate of the halogen compound gas while other depositionconditions are kept constant. However, the proportion can be changedwhen the input power, the reaction pressure, the shape of the dischargevessel and/or the introduction rate of the carbon productive gas arechanged. For instance, the variation of the conductivity when the inputpower is changed is plotted in FIG. 4. As shown in the diagram, theconductivity increases as the input power increases. Of course, otherdeposition conditions such as the NF₃ flow rate and C₂ H₄ flow rate weremaintained constant for plotting FIG. 4.

Another advantage of the present invention is low internal stressoccuring in films deposited. Dangling bonds have a tendency to increasethe stress. The dangling bonds can be terminated by hydrogen atomsintroduced into carbon films. However, even with the hydrogen atoms,some proportion of dangling bonds inevitably still remain withouttermination which might be a cause of internal stress. Thence, if ahalogen such as fluorine exists in the plasma gas, C--F bonds are easilycreated and, as a result, the density of dangling bonds is substantiallyreduced by the termination by fluorine atoms.

A further advantage of the present invention is high heat-resistantproperty.

A still further advantage of the present invention is low processtemperatures. At these low temperatures, carbon coating can be formedeven on selenium or organic materials such as plastics.

According to another aspect of the invention, a new CVD process isproposed which utilizes a mixed cyclotron resonance. In the improvedexciting process, sonic action of the reactive gas itself must be takeninto consideration as a non-negligible perturbation besides theinteraction between respective particles of the reactive gas and themagnetic field and microwave, and as a result charged particles of areactive gas can be energized in a relatively wide resonating space.Preferably, the pressure is maintained higher than 3 Torr. For the mixedresonance, the pressure in a reaction chamber is elevated 10² -10⁵ timesas high as that of the prior art. For example, the mixed resonance canbe established by increasing the pressure after ECR takes place at a lowpressure. Namely, first a plasma gas is placed in ECR condition at1×10⁻³ to 1×10⁻⁵ Torr by inputting microwaves under the existence ofmagnetic field. Then a reactive gas is inputted into the plasma gas sothat the pressure is elevated to 0.1 to 300 Torr and the resonance ischanged from ECR to MCR (Mixed Resonance).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical diagram showing the relation between theconductivity and the NF₃ rate in accordance with the present invention.

FIG. 2 is a graphical diagram showing the relation between thetransmissivity and the NF₃ rate in accordance with the presentinvention.

FIG. 3 is a graphical diagram showing the relation between the Vickershardness and the NF₃ rate in accordance with the present invention.

FIG. 4 is a graphical diagram showing the relation between theconductivity and the input power in accordance with the presentinvention.

FIG. 5 is a cross section view showing a CVD apparatus in accordancewith the present invention.

FIG. 6 is a graphical diagram showing the relation between thetransmissivity and the wavelength in accordance with the presentinvention.

FIG. 7 is a cross section view showing another CVD apparatus inaccordance with the present invention.

FIG. 8(A) is a graphical diagram showing a computor simulation of theprofiles of the equipotential surfaces of magnetic field in a crosssection.

FIG. 8(B) is a graphical diagram showing the strength of the electricfield of the microwave energy in the plasma generating space.

FIGS. 9(A) and 9(B) are graphical diagrams showing equipotentialsurfaces respectively in terms of magnetic field and electric fields ofmicrowave energy propagating in a resonating space.

FIG. 10 is a cross section view showing a further CVD apparatus inaccordance with the present invention.

FIG. 11 is a cross section view showing a still further CVD apparatus inaccordance with the present invention.

FIGS. 12, 13 and 14 are cross sectional views showing devices formed inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 5, a plasma CVD apparatus for depositing carbonmaterial on a surface in accordance with the present invention. Thesurface to be coated is for example made of glasses, metals, ceramics,organic resins and so forth.

The apparatus comprises a reaction chamber defining a reaction spacetherein, first and second electrodes 11 and 12, a high frequencyelectric power source 13 for supplying an electric power through amatching transformer 14, a DC bias source 15 connected in series betweenthe electrodes 11 and 12, a gas feeding system 1 consisting of fourpassages each of which is provided with a flow meter 7 and a valve 6, amicrowave energy supply 10 for exciting gases from the feeding system 1,a nozzel 9 through which gas excited by the microwave energy supply 10is introduced into the reaction space 20, and an exhausting system 16including a pressure control valve 17, a turbo molecular pump 18 and arotary pump 19. The electrodes are designed such that (the area of thefirst electrode 11)/(the area of the second electrode 12)<1.

In this apparatus, a carrier gas of hydrogen is introduced to thereaction space 20 from the gas feeding passage 2 as well as a reactivegas of a hydrocarbon such as methane or ethylene from the gas feedingpassage 3. In addition to this, a halogen compound gas such as NF₃ isinputted to the reaction space 20 through the gas feeding passage 4.Preexcitation may be effected by the microwave energy supply 10. Thepressure in the reaction space is maintained within the range between0.001 to 10 Torr, preferably 0.01 to 1 Torr. A high frequency electricenergy at a frequency not lower than 1 GHz, preferably 2.45 GHz, isapplied to the reactive gas at 0.1 to 5 kilo Watt for breaking C--Hbonds. When the frequency is selected to be 0.1 to 50 MHz, C═C bonds canbe broken and transformed to --C--C-- bonds. By virtue of this reaction,carbon atoms are deposited with a few halogen atoms in the form of astructure in which the diamond structure occurs at least locally.

A bias voltage of -200 to 600 V is set at the DC bias source 15. Theeffective bias voltage level is substantially -400 to +400 V since aself bias level of -200 V is spontaneously applied between theelectrodes 11 and 12 with the bias voltage level at the source 15 beingzero.

An experiment was carried out in the deposition conditions in which thehigh frequency input power was chosen between 50 Watt and 1 kilo Watt,e.g., 60 Watt, the pressure in the reaction space was 0.015 Torr, theflow rate of ethylene was 100 SCCM, the flow rate of NF₃ was 100 SCCM,the substrate temperature was room temperature, and the deposition timewas 30 minutes. This input power corresponds to 0.03 to 3 Watt/cm² interms of plasma energy. The transmissivity of the deposited carbon filmcontaining fluoride atoms were measured and plotted in FIG. 6. As shownin the diagram, the transmissivity was not lower than 95% at wavelengthslonger than 600 nm and not lower than 50% at a wavelength of 400 nm. Thevickers hardness was measured to be 1000 to 2500 kg/cm². The internalstress was measured to be as small as no higher than 10⁷ dyn/m². Nodeterioration was appreciable of the surface of the deposited film, whenexamined by means of an optical microscope at an 400 timesmagnification, after the surface was immersed for an hour in causticchemicals such as acids, alkalis, organic solvents and the like. Also,no appreciable change was observed after leaving the film in anincubator at 500° C. for an hour.

Referring to FIG. 7, an examplary microwave enhanced plasma CVDapparatus in accordance with the present invention is illustrated as asecond embodiment. As shown in the figure, the apparatus comprises areaction chamber in which a plasma generating space 21 and an auxiliaryspace 22 are defined which can be maintained at an appropriate negativepressure, a microwave generator 24, electro-magnets 45 and 45' in theform of Helmholtz coils surrounding the space 21, a power supply 45 forsupplying an electric power to the electro-magnets 25 and 25', and awater cooling system 38.

The plasma generating space 21 has a circular cross section, and withinthe plasma generating space, there is provided a substrate holder 30',made of a material which provokes minimum disturbance of magnetic fieldcreated by the magnets 25 and 25' in the chamber, e.g., made ofstainless steel or quartz. A substrate 30 is mounted on the holder 30'.The substrate holder 30' is irradiated and heated to 800°-1000° C. inthe atmosphere of a high temperature plasma gas by means of an infraredradiation 44, which is emitted from an IR heater 40, reflected from anIR reflection parabolic mirror 41 and focused on the back surface of theholder 30' through a lens 42. Reference numeral 43 designates a powersupply for the IR heater 40. Provided for evacuating the reactionchamber is an evacuating system comprising a turbo molecular pump 37 anda rotary pump 34 which are connected with the reaction chamber throughpressure controlling valves 31, 33 and 35. The substrate temperature mayreach a sufficient level solely by virtue of the plasma gas generated inthe reaction chamber and, in the case, the heater can be dispensed with.Further, depending on the condition of the plasma, the substratetemperature might become too high to enable a suitable reaction tooccur, in which case cooling means for the substrate has to be provided.

In use of the above described apparatus, a substrate 30 of silicon waferis mounted on a substrate holder 30', and the reaction chamber isevacuated to 1×10⁻⁶ Torr or a higher vacuum condition. Then, hydrogengas is introduced from a gas introducing system 26 at 100 SCCM to fillthe plasma generating space 21, and microwave energy at a power level of1 kilo Watts and a frequency of 2.45 GHz is injected from the microwavegenerator through a microwave introduction window 35 into the plasmagenerating space 21 which is subjected at the same time to a magnetic ofabout 2K Gauss generated by the magnets 25 and 25'. The magnets areadapted to adjust the magnetic field strength. The hydrogen is excitedinto a high density plasma state in the space 21. The surface of thesubstrate 30 is cleaned by high energy electrons and hydrogen atoms. Inaddition to the introduction of hydrogen gas, a productive gascomprising a hydrocarbon such as C₂ H₂, C₂ H₄, C₂ H₆, CH₃ OH, C₂ H₅ OHor CH₄ and a halogen compound gas such as CF₄, C₂ F₂, C₂ F₄, C₂ Cl₂ orC₂ Cl₄ are inputted at 30 SCCM (total rate) through a gas introductionsystem 27. The proportion of halogen compound gas is 50%. Theintroduction rate of a carrier gas (hydrogen) is selected between 30 and0, for example 2.

Since chlorine is somewhat corrosive, fluorine is most suitable in viewof easiness of handling and highly reactiveness to hydrogen. Thereactive gas and the carrier gas are perferably non-oxide gases since,if water is composed, strong acides such as HF and HCl might becomposed.

Then, carbon atoms excited to high energy states are generated at150°-500° C., and deposited on the substrate 30 on the substrate holder30' in the form of a film of 0.1 to 100 microns thickness containingfluoride. The pressure of the reactive gas is 3-800 Torr, preferably apressure not lower than 10 Torr such as 10-760 Torr. The carbon film hada smooth surface and was abrasion-proof and corrosion-proof, andtherefore suitable for applications to instruments for chemicalexperimental.

In addition to the above described reactive gas, Ni(CO)₄ at 1 SCCM (andGeH₄ at 2 SCCM in addition, if necessary) as a catalyst may be inputtedto the reaction chamber from the introduction system, the proportion ofthe catalyst to the carbon compound gas being 0.1% to 10%. NiF, NiO,NiF(H₂ O)_(n) (where n=1.3), Ni(CN)₂, Ni(C₅ H₅)₂, GeH₄, GeF₄, manganesecarbonyl, MnF₂ and the like are examples of other catalysts. They can beused respectively alone or combinations. The CVD reaction which occursresults from carbon atoms being excited to a high energy condition andheated to 150°-500° C. by virtue of the plasma gas and the heater 40 sothat the substrate 30 mounted on the substrate holder 30' is coated withcarbon in the form of a 0.1-100 microns thick film of i-carbon(insulated carbon consisting of micro-crystals) or diamond having agrain diameter of from 0.1 to 100 microns. In accordance withexperimental, it took only two hours to deposit a carbon film having anaverage thickness of 5 microns. The deposition speed can be increased byapplying a bias voltage to the substrate holder. The carbon product inaccordance with the present invention is characterized that at least 50%of carbon atoms have been connected by sp³ bonds.

For reference, a film formation process was performed in the same manneras in the above but without using a catalyst. As a result, it took 15hours to form a carbon film having an average thickness of 4 microns. Itwas confirmed by a metal microscope (1000 times magnification) that theunevenness of the surface of the film was significant. In accordancewith the present invention, since innumerable seeds of catalyst prevailover the surface to be coated, carbon films can be formed with flatsurfaces.

In case using the reactive gas of CH₄ :CF₄ =2:1, thin films containingdiamond structures therein could be formed at 100 Torr at a temperaturenot lower than 400° C. In case of CH₄ :CF₄ =1:1, the formationtemperature was 300° C. and the reaction pressure was 50 Torr. In caseof CH₄ :CF₄ =1:2, thin films containing diamond structures could beformed at 200° C. and 10 Torr. While the pressure is preferably aroundatmosphere pressure, low temperatures as low as 500° C. is desirablebecause the choice of substrates to be coated becomes broad.

At temperatures not higher than 400° C., carbon deposition can beperformed on a semiconductor substrate over an aluminum circuit formedthereon. AT temperatures not higher than 200° C., carbon coating on aplastic substrate becomes possible. At temperatures between 300° C. and500° C., carbon coating can be deposited on glass substrates.

FIG. 8(A) is a graphical showing of the distribution of the magneticfield in the region 30 in FIG. 7. The curve in the diagram are plottedalong equipotential surface and are given numerals indicating thestrengths along the respective curves of the magnetic field induced bymagnets 5 and 5' having a power of 2000 Gauss. By adjusting the power ofthe magnets 5 and 5', the strength of the magnetic field can becontrolled so that the magnetic field becomes largely uniform over thesurface to be coated which, is located in the region 100 where themagnetic field (875±185 Gauss) and the electric field interact. In thediagram, the reference 26 designates the equipotential surface of 875Gauss at which the conditions required for ECR (electron cyclotronresonance) between the magnetic field and the microwave frequency aresatisfied. Of course, in accordance with the present invention, ECR cannot be established due to the high pressure in the reaction chamber, butinstead a mixed cyclotron resonance takes place in a broad regionincluding the equipotential surface which satisfied ECR conditions. FIG.8(B) is a graphical diagram in which the X-axis corresponds to that ofFIG. 2(A) and which shows the strength of the electric field of themicrowave energy in the plasma generating space 1. As shown, theelectric field strength attains its maximum value in the regions 100 and100', it is difficult to heat the substrate 10' without disturbing thepropagation of the microwave energy. In other regions, a film will notbe uniformly deposited, but will be deposited in the form of a doughnut.It is for this reason that the substrate 10 is disposed in the region100. The plasma flows in the lateral direction. According toexperiments, a uniform film can be formed on a circular substrate havinga diameter of up to 100 mm, and a film can be formed in the chamber on acircular substrate having a diameter of up to 50 mm with a uniformthickness and a uniform quality. When a larger substrate is desired tobe coated, the diameter of the space is doubled with respect to thevertical direction of FIG. 8(A) by making use of 1.225 GHz as thefrequency of the microwave energy. FIG. 9(A) and FIG. 9(B) are graphicaldiagram showing the distribution of the magnetic field and the electricfield due to microwave energy emitted from the microwave generator 4 fora cross section of the plasma generating space 1. The curves in thecircles of the figures are plotted along equipotential surfaces andgiven numerals showing the field strengths. As shown in FIG. 9(B), theelectric field reaches its maximum value at 25 KV/m.

Next, a third embodiment will be described referring to FIGS. 10 and 11.A large size plasma treatment system comprises a reaction chamber 57provided, with a loading chamber 57-1, and unloading chamber 57-2, apair of guide rails 59 for suspending therefrom a plurality of substrateholders 52 made of aluminium or nickel plates, a high frequency electricpower source 65 for supplying an electric power through a matchingtransformer 66, first and second metallic mesh electrodes 53 and 53'connected to the output terminals 54 and 54' of the transformer 66, thegeometric area of each electrode being 150 cm² and the effective area ofeach electrode being 120 cm², an alternating electric power source 67connected between the midpoint of the secondary coil of the transformerand the guide rails 59, a gas feeding system 60 consisting of fourpassages each of which is provided with a flow meter 57 and a valve 56,a nozzel 75 for inputting gases to the reaction chamber 57 from the gasfeeding system 60, and an exhausing system 70 including a pressurecontrol valve 71, a turbo molecular pump 72 and a rotary pump 73. Areaction space is defined within the reaction chamber by a four-sidedhollow structure 58 and 58' of 160 cm width, 40 cm depth and 160 cmheight for blocking deposition on the inside wall of the reactionchamber 57. The height of the hollow structure may be chosen between 20cm and 5 m in general. One dimmension of the electrodes 53 and 53' maybe chosen between 30 cm and 3 m in general. There are provided gatevalves 64-1 and 64-4 between the outside and the loading and unloadingchambers 57-1 and 57-2 and gate valves 64-2 and 64-3 between thereaction chamber 57 and the loading and unloading chambers 57-1 and 57-2for sealing off. The inside of the reaction chamber 57 is provided witha heater consisting of a plurality of halogen lamps 61 and 61'.

A plurality of substrates 51-1, 51-2, . . . 51-n are mounted on theplurality of substrate holders 52-1, 52-2, . . . 52-n. The distances81-1, 81-2, . . . between each adjacent holders in the reaction chamber57 are selected substantially constant, the dispersion from the averagebeing within ±20%. The corresponding distances in the loading chamberare selected more narrower for the purpose of designing the systemcompact. In this arrangement, only one side, surface of each substrateis coated. If coating of both surfaces is desired, the substrates aresupported in openings formed on the holders. Introduced to the reactionchamber 57 are a carrier gas of argon or hydrogen from the passage 60-1of the gas feeding system, a reactive gas of a hydrocarbon such asmethane or ethylene from the passage 60-2 and a halogen compound gassuch as NF₃ from the passage 60-3. The pressure of the reactive gas is0.001 to 1.0 Torr, e.g. 0.05 Torr. The substrate temperature is -100° C.(in case with a cooling system) or up to 150° C.

A first alternating voltage is applied between the mesh electrodes 53and 53' at a high frequency of 1 MHz to 5 GHz, e.g. 13.56 MHz while asecond alternating voltage is applied between the midpoint of thesecondary coil and the rails 59 at a frequency of 1 KHz to 500 KHz, e.g.50 KHz. The input power of the first alternating voltage is 1.0 KW to 30KW (equivalent to a plasma energy of 0.04-1.3 KW/cm²), e.g. 10 KW(equivalent to a plasma energy of 0.44 W/cm²). The second alternatingvoltage functions to apply a AC bias voltage of -200 to 600 V(equivalent to 500 W) at the substrate surface. By virtue of thiselectric power, a plasma gas is generated in the reaction chamber 57 andinitiates chemical vapor reaction. The exhausted gas is removed throughthe evacuation system 70.

In what follow, experimental results will be described in details whichhave been obtained in accordance with the aforemensioned embodiments andcombination thereof.

EXPERIMENT 1

In accordance with the present invention, a photosensitive structure foruse in electron photography was formed as illustrated in FIG. 12. Thestructure comprises a PTE sheet 101 of 20 microns thickness, anAluminium film 102 of 600 Å thickness formed by vacuum evaporation, anintermediate film 103, a charge generating film 1-4 of 0.6-1.2 micronsthickness, a charge transfer film 105 of 20 microns thickness and apassivation film 106 formed in accordance with the present invention.

If the passivation film or the charge generating film is given anegative electric charge, some portion of the structure receiving lightrays 107 can be neutralized by holes generated in the charge generatingfilm 104 which reach the passivation film 106 through the chargetransfer film 105. In this case, electrons generated in the chargegenerating film 104 are drained through the intermediate film and thealuminium film. Other region which does not receive light rays attranctstoner and transfers it to the surface of a paper to construct an imagein accordance with the existence or absence of incident light rays.

The resistivity of the passivation film 106 was controlled to be 10¹¹ to10⁹ ohm cm by adjusting the flow rate of NF₃. By virtue of such a highresistivity, it was avoided that the boundary of the constructed imagebecame out of focus due to lateral drift of charge. Accordingly, theformed image became clear with high contrast. On the other hand, if theresistivity were too high, residual charge might be accumulated afterrepeating charge and discharge. This adverse accumulation could beavoided by precisely controlling the resistivity in accordance with thepresent invention. The transmissivity of the passivasion film was notlower than 80% to light rays of 500 nm or longer wavelengths, and notlower than 60% to light rays of 400 nm or longer wavelength.Accordingly, the structure can be employed for application making use ofwavelengths of visible lights.

Furthermore, the passivation film was abrasion-proof andscratching-proof and superior in adhesivity with lower internal stress.No crack and no peeling was caused in the photosensitive film on theflexible sheet even when the sheet was bended at a radius of cavature of10 mm. This experiment was made for application to a sheet-like organicphotosensitive structure. However, similar structures can be formed inthe same manner as photosensitive drums, amorphous siliconphotosensitive structures and selenium photosensitive structures.

This Example was repeated in the same manner with the followingexception. The passivation film in this case was composed of two layers.The lower layer was deposited at a NF₃ flow rate not higher than 0.1SCCM for begining two minutes and then the upper layer was deposited at100 SCCM for the subsequence 20 minutes. The resistivity, thetransparency, the hardness, the internal stress and the othercharatcteriscis were depending almost on the property of the upperlayer. Only the adhesivity depended on the lower layer which made thesturdy mechanical contact. The resistivity was measured to be 10¹¹ to10⁹ ohm cm.

Another layer may be further deposited over the upper layer with a fewerfluoride proportion to rise the hardness of the external surface of thepassivation film.

EXAMPLE 2

The present invention was applied to film coating on an IC chip afterwire bonding on lead structure for reliability. The carbon film wascomposed of two layers. The lower layer was deposited to a thickness of0.6 micron at a flow rate such that C₂ H₂ /NF₃ =1:1 whose internalstress was measured to be 10⁷ dyn/cm. The upper layer was deposited to0.1 micron at flow rate such that C₂ H₂ /NF₃ =100:1 whose vickershardness was measured to be 2000 kg/cm².

The small internal stress in accordance with the present invention waseffective for avoiding disconnection of gold wire from alumnium pads dueto internal stress, along with another advantage that the lower layerprevents water and alkaline ions from entering into the structure.

EXAMPLE 3

A representative structure of a thermal printer head is illustrated inFIG. 13. A glaze film 102 was formed on an insulating substrate 101 witha protruding glaze 103 which was to be a heat generating part. On thisstructure, a heating film 105 and an electrically conductive film 106were laminated sequentially followed by photolithography to form awindow corresponding to a heat generating element 121. Then, a carboncontaining passivasion film 107 was formed over the upper surface of thestructure in accordance with the present invention.

While conventional passivasion films are inorganic films such as siliconnitride films as thick as 5 microns, the passivasion films, inaccordance with this invention, can be as thin as 1 micron since thehardness can be controlled not lower than a Vickers hardness of 2000kg/mm² by adjusting the flow rate of NF₃.

Furthermore, the internal stress of the films formed in accordance withthe present invention was about 10⁹ dyn/cm² and the property of the filmwas not degraded even after leaving at 100° C. in air for an hour. Theresistivity on the order of 10¹⁰ ohm cm is suitable for avoiding staticelectricity, and as a result, dust can be eliminated which may be, causeof scratchs or mulfunction of electric circuits. The heat generatingfilm may be made of conventional materials or made of a carbon filmwhose halogen proportion is controlled so that the conductivity is 10³to 10⁴ ohm cm.

EXAMPLE 4

FIG. 14 is a partial view showing a contact image sensor formed inaccordance with the present invention. A photosensor 134 was formed on atransparent glass substrate 133 by patterning an amorphous silicon filmand conductive films formed by a known CVD making use of an eximerlaser. A transparent polyimide film 135 was formed on the substrate 133over the sensor 134. A passivation film 136 was deposited on thepolyimide film 135 to a thickness of 2.0 microns.

The Vickers hardness was measured to be 2500 Kg/mm² and the resistivitywas measured to be 1×10⁵ ohm cm. Since the carbon passivasion film 136has a highly hardness and an improved insulating capability which arecomparable to those of diamond. For this reason, the surface hassufficient resistivity against abrasion due to unevenness of a paper ordue to a staple.

In accordance with the present invention, improved carbon films orclusters can be formed. The effect of the invention has been confirmedin regard to carbon deposition, and therefore it is advantageous toapply the present invention to the formation of any films containingcarbon whose proportion is not lower than 50%.

While a description has been made for several embodiments, the presentinvention should be limited only by the appended claims and should notbe limited by the particular examples, and there may be caused toartisan some modifications and variation according to the invention. Forexample, it has been proved effective to add boron, nitrogen, phosphorusor the like into the carbon.

We claim:
 1. A method of depositing a material constituting mainly ofcarbon comprising the steps of:placing an object to be coated in thereaction space of a reaction chamber; introducing a reactive gascomprising carbon into said reaction space; inputting electric energy tosaid reaction space by means of a pair of electrodes in order to provideplasma and decompose said reactive gas and deposit said material on thesurface of said object where said object is placed on a holder spacedfrom each of said pair of electrodes, wherein a bias voltage is appliedto said object for enhancing the hardness of the deposited materialvoltage.
 2. The method of claim 1 wherein said electric energy issupplied from the respective ends of a secondary coil of a transformer,where the midpoint of the secondary coil is grounded.
 3. The method ofclaim 2 wherein the frequency of said electric energy is 1 MHz to 5 GHz.4. The method of claim 1 wherein said bias voltage is an AC biasvoltage.
 5. The method of claim 4 wherein the frequency of said AC biasvoltage is 1 KHz to 500 KHz.
 6. The method of claim 2 where said biasvoltage is applied to said object from a voltage source connectedbetween the grounded midpoint of the secondary coil and the object to becoated.
 7. A method of depositing a carbonaceous material comprising thesteps of:placing an object to be coated between a pair of electrodes ina reaction chamber; introducing a reactive gas comprising carbon intosaid reaction chamber; inputting a first voltage to said pair ofelectrodes in order to convert said reactive gas to plasma; anddepositing said carbonaceous material on said object, wherein saidobject is placed on a substrate holder spaced apart from said pair ofelectrodes and wherein said substrate holder is supplied with a secondvoltage in order to induce a bias voltage on said object for enhancingthe hardness of the deposited carbonaceous material.
 8. The method ofclaim 7, wherein said first voltage is a high frequency voltage.
 9. Themethod of claim 8, wherein a frequency of said second voltage is lowerthan that of said first voltage.